1. Field of the Invention
This invention relates to a Minimization of Drive Test (MDT) information reporting format and a time stamp configuring method to support a MDT function in wireless communication systems, or 3rd Generation Partnership Project (3GPP) systems.
The method defines a reporting format needed in a process where user equipment (UE) transmits MDT measurements to a serving base station, and sets the number of bits required to log time stamp and the unit of logging time.
2. Description of the Related Art
Mobile communication systems have been developed to provide communication services to users while they are moving. With the rapid development of communication technology, mobile communication systems have provided rapid data communication services as well as voice communication services. The specification for Long Term Evolution-Advanced (LTE-A), one of the next generation mobile communication systems, is being standardized by the 3GPP. The standardization for LTE-A is proceeding to be completed by the latter half of 2010. LTE-A refers to a technology that can provide packet-based communication with a higher transmission rate than the current data transmission rate.
As the 3GPP standardization is evolved, convenient optimizations for wireless networks are being discussed other than increases in communication service speed. When establishing wireless networks at the initial stage or optimizing networks, base stations or base station controllers must collect wireless environment information for their cell coverage, which is called a ‘drive test’. Conventional drive tests are executed in such a way that engineers load measurement devices on a vehicle and repeatedly perform a measurement task for a relatively long period of time, which is complicated. The measurement results are processed in analysis processes and they are used to set system parameters of each of base stations or base station controllers. Conventional drive tests increase in wireless optimization cost and management cost and consume significant time. Therefore, in order to minimize drive tests and to improve analysis processes for wireless environments and manual setting processes, a study called ‘Minimization of Drive Test (MDT)’ is made. To this end, instead of performing Drive Test, UE measures a channel quality and transmits measurements regarding a corresponding wireless channel to a base station periodically or immediately when an event occurs. Alternatively, after a certain period of time has elapsed from the storage of wireless channel measurements, UE may transmit measurements regarding a corresponding wireless channel to a base station. In the following description, the transmission of wireless channel measurements measured by UE and additional information to a base station is called an MDT measurements reporting operation. In this environment, if UE is capable of communicating with a base station, it can immediately transmit the channel quality measurement result to the base station. On the contrary, if UE isn't capable of performing an immediate report, it logs MDT measurements and then reports, when it can communicate with the base station, the information to the base station. The base station uses the received MDT measurements to optimize cell coverage. LTE-A classifies types of MDT measurements reporting, according to UE RRC states, as the following table 1.
TABLE 1UE RRC stateMDT measurements reporting by UEIdle modeLogging and deferred reportingConnected modeImmediate reporting
As described in table 1, ‘idle mode’ refers to a state where UE is not communicating with a base station, and ‘connected mode’ refers to a state where UE is communicating with a base station. While MDT is performed, channel quality information measured by UE can be transmitted via RRC signaling. Therefore, although UE operates in an idle mode, it can switch the idle mode to a connected mode in order to transmit corresponding information. In that case, UE logs channel measurements and defers the transmission to a base station until its idle mode is switched to a connected mode.
MDT measurements are transmitted to the base station by the use of control plane protocol stacks for processing RRC and NAS signals.
FIG. 1 illustrates a general control plane protocol stack of an LTE system. RRC layers 105 and 155 perform system information transmission, RRC link control, channel measurement control, etc. PDCP 110 and 150 compress/decompress IP header. RLC 115 and 145 reconfigure PDCP PDU in proper size and perform an ARQ operation. MAC 120 and 140 connect to a number of RLC devices configured in one user equipment. MAC 120 and 140 multiplex RLC PUDs to MAC PDU, and de-multiplex RLC PDUs from MAC PDU. The protocol layer devices include proper headers at need. For example, RLC device adds an RLC header including a serial number, etc., to RLC SDU. MAC device adds an MAC header including an RLC device identifier, etc., to MAC SDU. Physical layers (PHY) 125 and 135 in UE and ENB channel-code and modulate MAC PDU, create OFDM symbols, and transmit them via a wireless channel 130. PHY 125 and 135 demodulate and channel-decode OFDM symbols received via a wireless channel 130, and transfer them to the upper layers. PHY 125 and 135 also perform Hybrid Automatic Retransmission request (HARQ) for MAC PDU. HARQ performs retransmission on PHY layer and is a soft combination of a retransmitted packet and the original packet.
As shown in FIG. 2, MDT measurements logged by UE is transferred from RRC layer to PDCP layer via Signaling Radio Bearer 2 (SRB2). RRC control message or NAS message 210 is transferred from RRC layer 205 to PDCP layer 230 via SRB0 (215), SRB1 (220) or SRB2 (225). SRB0 is used to transfer RRC message via CCCH. SRB0 has the highest priority order. SRB1 is used to transfer RRC message via DCCH, and also NAS message part of which is transferred in piggyback format. SRB2 is used to transfer NAS message via DCCH. The packets transferred by SRB1 and SRB2 are all encoded via integrity and ciphering processes. SRB1 has a higher priority order than SRB2. MDT measurements are transferred via SRB2 with the lowest priority order. User plane data can be also transferred via Data Radio Bearer (DRB) 250 other than SRB0˜SRB2. Packets transferred via DRB are processed by a ciphering and ROHC process 255 and transferred to RLC layer 265. RLC layer maps corresponding packets to DTCH.
UE operating in idle mode logs MDT measurements periodically or when the measured information meets a specific event. An example of an event for logging MDT measurements is as follows.                (1) Periodical downlink pilot measurements        (2) Serving Cell becomes worse than threshold        (3) Transmit power headroom becomes less than thresholdLogged MDT Measurements:        (1) Global cell ID of a serving cell        (2) Measurements regarding Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) of a serving cell        (3) Location information        (4) Time stamp        
MDT measurements need to include information to identify a serving cell, i.e., global cell ID. This tells which cell corresponding information is acquired. A global cell ID must uniquely indicate one cell. A wireless channel state of a serving cell can be expressed via a specific measurement. The states of EUTRA can be expressed via RSRP and RSRQ, UTRAN via RSCP and Ec/No, and GERAN via Rxlev. The embodiments of the invention are described based on EUTRA LTE systems; however, it should be understood that the invention can also be applied to the other systems. In 3GPP, the MDT function will be applied to LTE and UMTS.
Location information from among MDT measurements transmitted to a base station serves as an important component. If UE can't acquire the GPS-based location information, it measures a set of Reference Signal Received Power from the adjacent base stations and informs the base station of it. The set of Reference Signal Received Power is called a RF fingerprint. Since the base station receiving the RF fingerprint has known the location information regarding the adjacent base stations, it applies the signal power values of the adjacent base stations to a signal path attenuation model and thus predicts the distances between the UE and the respective, adjacent base stations. If the location information regarding the adjacent base stations and the predicted distances between the UE and the adjacent base stations are applied to a technique, such as a triangulation, an approximate location of the UE can be acquired. If UE isn't capable of acquiring the GPS-based location information, it can acquire information regarding a predicted location instead of an accurate location and transmit it to the base station.
Time stamp is also important to acquire MDT measurements. When wireless channel measurement is performed, time stamp serves to optimize service coverage. This is because wireless channel states vary according to hours. Time stamp is more importantly used in a logging and deferred reporting operation in an idle mode than an immediate reporting operation in a connected mode. Since an immediate report in a connected mode is going to include a result measured immediately before the reporting operation, time stamp isn't important in the immediate reporting operation in a connected mode. However, if a logging and deferred reporting operation in an idle mode doesn't use time stamp, it can't be predicted when the measurement was performed. Therefore, the currently developed 3GPP standard doesn't have time stamp for an immediate reporting operation in a connected mode; however, it includes time stamp as necessary information in a logging and deferred reporting operation in an idle mode.
Time stamp can be provided in various types. Types of time stamps refer to an absolute time or a relative time that UE can provide. An absolute time needs a number of bits to report a corresponding time stamp. On the contrary, a relative time may need a relatively small number of bits than the absolute time.
In the 3GPP standard, relative time stamp is included in MDT measurements in order to reduce signalling overhead. When base stations provide reference information regarding absolute times to UEs, the UEs include relative time stamps in respective measurement samples, based on the received absolute times. After completing MDT measurement, UE reports the logged measurements to a base station and also informs the base station of the absolute time reference information that it has received from the base station. This is because a base station that provided absolute time reference information to UE at the initial stage may differ from a base station that the UE intends to make a report to.
FIG. 3 illustrates a flow chart describing a method for performing MDT measurement in a logging and deferred reporting operation in an idle mode. eNB 305 configures MDT measurement and transmits corresponding information, i.e., MDT measurement configuration information to UE 300 in a connected mode (310). The MDT measurement configuration information includes absolute time reference information, sampling cycle, measurement duration, etc. The absolute time reference information has been already described above. The sampling cycle is used for periodical downlink pilot measurements, and also used to measure wireless channels at each cycle. The measurement duration refers to the total period of time in performing MDT. UE performs MDT until a corresponding measurement duration has elapsed.
If the RRC connected state of UE 300 with the eNB 305 is switched from a connected mode to an idle mode, the UE 300 starts to perform MDT measurement (315). UE 300 measures and logs MDT (320), and then continues measuring and logging MDT at the sampling cycle that was received (325). MDT measurements are logged at each measurement cycle (330).
If UE 300 enters a connected mode (335), it informs the eNB 305 as to whether it has logged MDT measurements (340). eNB 305 may request a report from the UE 300 according to a corresponding state. If UE 300 receives the request from eNB 305, it reports the MDT measurements that it has logged by the time to the eNB 305, and then removes it. On the contrary, if UE 300 doesn't receive the request from eNB 305, it retains the logged information. If UE 300 enters an idle mode but the measurement duration hasn't elapsed (345), it continues to perform MDT measurement and acquires the MDT measurements (350). Measurement duration may or may not take a period of time in a connected mode into account. If the measurement duration has elapsed (355), UE 300 stops MDT measurement.
UE 300 enters a connected mode (360). UE 300 informs the eNB 305 that there is a newly logged MDT measurement, and reports, if the request is made from the eNB 305, it to the eNB 305 (365).
However, in order to efficiently transmit MD measurements from UE to eNB, the MDT information reporting format to the eNB and the time stamp configuration must be defined.